move nullable check to variable input evaluation

internal/terraform/evaluate.go

@@ -277,18 +277,6 @@ func (d *evaluationStateData) GetInputVariable(addr addrs.InputVariable, rng tfd
 	case val.IsNull() && !config.Nullable && config.Default != cty.NilVal:
 		// If nullable=false a null value will use the configured default.
 		val = config.Default
-
-	case val.IsNull() && !config.Nullable:
-		// The value cannot be null, and there is no configured default.
-		diags = diags.Append(&hcl.Diagnostic{
-			Severity: hcl.DiagError,
-			Summary:  `Invalid variable value`,
-			Detail:   fmt.Sprintf(`The resolved value of variable %q cannot be null.`, addr.Name),
-			Subject:  &config.DeclRange,
-		})
-		// Stub out our return value so that the semantic checker doesn't
-		// produce redundant downstream errors.
-		val = cty.UnknownVal(config.Type)
 	}
 
 	var err error

internal/terraform/node_module_variable.go

@@ -253,7 +253,23 @@ func (n *nodeModuleVariable) evalModuleCallArgument(ctx EvalContext, validateOnl
 		val = cty.UnknownVal(n.Config.Type)
 	}
 
+	// If there is no default, we have to ensure that a null value is allowed
+	// for this variable.
+	if n.Config.Default == cty.NilVal && !n.Config.Nullable && val.IsNull() {
+		// The value cannot be null, and there is no configured default.
+		diags = diags.Append(&hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  `Invalid variable value`,
+			Detail:   fmt.Sprintf(`The resolved value of variable %q cannot be null.`, n.Addr),
+			Subject:  &n.Config.DeclRange,
+		})
+		// Stub out our return value so that the semantic checker doesn't
+		// produce redundant downstream errors.
+		val = cty.UnknownVal(n.Config.Type)
+	}
+
 	vals := make(map[string]cty.Value)
 	vals[name] = val
+
 	return vals, diags.ErrWithWarnings()
 }